1. Field of the Invention
The present invention relates to a method of measuring a magnetic write width in discrete track recording for writing data on a discrete track medium using a magnetic head.
2. Description of the Related Art
In order to satisfy demand for higher recording density of a magnetic recording and reproducing apparatus, particularly of a magnetic disk drive apparatus, it is requested to more improve performance of a thin-film magnetic head and a magnetic recording medium.
As for a thin-film magnetic head, a composite type thin-film magnetic head with a laminated structure of a magnetoresistive effect (MR) read head element and an electromagnetic transducer write head element is widely utilized. The electromagnetic transducer write head element now starts to adopt a perpendicular magnetic recording structure capable of providing finer recording bits on a magnetic recording medium.
On the contrary, as for a magnetic recording medium, a structure adapted for the above-mentioned perpendicular magnetic recording method, particularly a discrete track structure with tracks physically separated to each other is getting to attract attention. In general, each recording bit is composed of a plurality of fine magnetic particles. Therefore, in order to increase a recording density, it is necessary to reduce a diameter of the fine magnetic particles to minimize microscopic asperities on the boundary of the recording bits. However, if the diameter of the fine magnetic particles is reduced, thermal stability in magnetization of the recording bits will deteriorate due to the decreased volume of the fine magnetic particles. The aforementioned perpendicular magnetic recording structure can solve this problem of the thermal stability in magnetization.
When a size of the recording bit becomes small and therefore a width of the written track becomes narrow, magnetic interference from the neighboring tracks may increase. A discrete track medium has fine grooves for physically separating the neighboring tracks to each other to reduce possible magnetic interference. Thus, this discrete track medium can extremely increase the surface recording density. In fact, discrete tracks with a track pitch of 90 nm for example are realized by using an ultra micromachining process such as an electron-beam lithography process or a precise dry-etching process.
In discrete track recording (DTR) for writing data onto the discrete track medium by using a magnetic head, it is very important to precisely measure a magnetic write width of the magnetic head (MWWHEAD) and a magnetic write width of the discrete track medium (MWWMEDIUM) and to use the measured widths for designing a magnetic head and a discrete track medium with desired performance. The magnetic write width of the magnetic head MWWHEAD is different from a physical width of a write pole of the magnetic head but defined as a width of a write pole region within which a write magnetic field effectively operating on the medium is produced during actual write operation. Also, the magnetic write width of the discrete track medium MWWMEDIUMis different from a physical width of a track of the medium but defined as a width of a medium region in the track-width direction, within which a signal magnetic field read out by an MR read head element and contributed to an actual read output is produced.
Indeed, in a conventional magnetic recording medium, it is important to measure these magnetic write widths MWWHEAD and MWWMEDIUM. For example, U.S. Pat. No. 6,680,609 B1 discloses a method of determining a magnetic track width of a conventional magnetic head with a magnetic read width MRW narrower than a magnetic write width MWWHEAD (MRW<MWWHEAD). Also, U.S. Pat. No. 7,119,537 B2 discloses a method of directly measuring a magnetic read width MRW from the derivative of a full track profile and a method of obtaining a magnetic write width MWWHEAD. Further, U.S. Pat. No. 6,608,477 B2 discloses a method of qualifying a head by producing a modified track scan data based on a difference between a magnetic write width MWWHEAD and a nominal track width. Still further, Japanese Patent Publication No. 2000-11336A discloses a magnetic disk apparatus with a relationship of E≧R≧W, where E is a signal erasing width of an electromagnetic transducer element, R is a reproducing track width of an MR head, and W is a signal recording width of the electromagnetic transducer element.
In the conventional head, it is designed to have a magnetic read width MRW narrower than a magnetic write width MWWHEAD (MRW<MWWHEAD) as aforementioned. However, according to the discrete track recording (DTR) system, it is impossible to measure a magnetic write width of the discrete track medium MWWMEDIUM using this configuration of the conventional head.
Namely, according to the conventional magnetic recording system, since some off-track is allowed to shorten a seek time or to suppress runouts of the spindle motor, there exists background data that are data on non-overwritten area along both sides of the data track. In order to avoid reading of such background data and of data on the adjacent track, the conventional head is designed as MRW<MWWHEAD.
Contrary to this, according to the DTR system, there exist grooves along both sides of the data track, for keeping a predetermined distance from the adjacent track, and therefore no background data exists on the medium. As a result, it is possible to design the DTR system to have a magnetic read width MRW wider than a magnetic write width MWWHEAD (MRW>MWWHEAD), and in fact it is so implemented. If the magnetic read width MRW is widened, a height of a magnetic sensitive part of the MR element in a direction perpendicular to an air bearing surface (ABS) can be increased, so that the fabrication of the MR element becomes easy. Particularly, in case of a current perpendicular to plane type magnetic head, its element resistance can be reduced.
Zhong-Heng Lin, et al., “Full-Track Profile Derivative Method for Track Width Measurements of Magnetic Recording Head”, IEEE TRANSACTIONS ON MAGNETICS, Vol. 41 No. 10, October 2005, pp.3067-3069 discloses that, when a magnetic recording system is designed to have a magnetic read width MRW wider than a magnetic write width MWWHEAD (MRW>MWWHEAD), an error in measurement of MWWHEAD will increase depending upon the reduction of a true value of MWWHEAD. The measured value MWWHEAD will converge to the MRW value when the true value of MWWHEAD decreases and thus the difference between the measured value and the true value of MWWHEAD will increase. It is possible to calculate a magnetic write width of the medium MWWMEDIUM even under the condition of MRW>MWWHEAD, by using the method described in Zhong-Heng Lin, et al., “Full-Track Profile Derivative Method for Track Width Measurements of Magnetic Recording Head”, IEEE TRANSACTIONS ON MAGNETICS, Vol. 41 No. 10, Oct. 2005, pp.3067-3069. However, due to the influence of possible distortion in the track profile, measurement of the magnetic write width MWWHEAD is quite difficult. In fact, under the condition of MRW>MWWHEAD, since it is impossible for the read head element to provide its maximum output, a large error will occur in a value MWWHEAD calculated from the average track amplitude (TAA).
In general, on the discrete track medium, it is difficult to leave traces corresponding to MWWHEAD. Therefore, due to the aforementioned facts, in the DTR system, it is extremely difficult to accurately measure a magnetic write width of the magnetic head MWWHEAD and a magnetic write width of the discrete track medium MWWMEDIUM.